In many wireless communication systems, it is indeed necessary to listen to the communication channel, in order to check the availability of it, before sending a message on said communication channel.
This is the case in particular for wireless communication systems that use so called unlicensed frequency bands in that they can be used without any prior administrative authorisation, as long as the regulatory constraints are complied with. These regulatory constraints comprise indeed, in certain geographical zones, the obligation to listen before talking (LBT).
In such wireless communication systems, it is therefore possible to send a message only if no activity has been detected on said communication channel for a predetermined time, termed the “inactivity time”. For example, in Japan, the inactivity time to be detected before sending a message must be at least 5 milliseconds (ms).
In the current wireless communication systems, a terminal that wishes to send a message on such a communication channel therefore begins by listening to said communication channel. If the communication channel is busy, or if an activity is detected before said inactivity time was able to be observed, the terminal can immediately interrupt the listening, and try later to listen to said communication channel after expiration of a predetermined time, termed the “listening delay time”. The terminal can also prolong the listening until it detects an absence of activity on the communication channel during said inactivity time. As soon as an absence of activity for a time equal to said inactivity time has been detected, the terminal can send the message.
Using unlicensed frequency bands is particularly advantageous in the case of applications of the M2M (“Machine-to-Machine”) type or of the “Internet of Things” (IoT) type, as it makes it possible to reduce the costs of such wireless communication systems.
In the context for example of the IoT, each object of everyday life has a vocation of becoming a communicating device, and is to this effect equipped with a terminal suitable for sending messages intended for an access network. As such, in such a context, it is entirely possible that a very large number of terminals, typically greater than 100 and even more, share the same communication channel.
A problem with known techniques of listening before sending resides in the fact that, in the case of a large number of terminals sharing the same communication channel, it can be difficult to access the communication channel. Indeed, the probability of having a terminal that sends a message at the instant when another terminal is listening to the communication channel increases with the number of terminals, in such a way that the terminals spend a lot of time waiting for the communication channel to be free.
Furthermore, the fact that it is sufficient that at least one terminal is sending a message in the communication channel so that the latter is considered as busy, independently of the possible faculty of the terminals to use said communication channel without interfering between them, leads to a use that is not very effective of said communication channel.